The production of terephthalic acid (TA) typically involves the liquid phase oxidation of para-xylene (PX) feedstock using molecular oxygen in acetic acid as a process solvent, in the presence of a dissolved heavy metal catalyst system usually incorporating a promoter, such as bromine as disclosed in U.S. Pat. No. 2,833,816. In general, acetic acid, molecular oxygen in the form of air, para-xylene and catalyst are fed continuously into the oxidation reactor at elevated temperature and pressure, typically a temperature from about 150° C. to about 250° C. and a pressure from about 100 kPa to about 5000 kPa.
Para-xylene oxidation produces a high-pressure gaseous stream (or “off-gas”) which comprises nitrogen, unreacted oxygen, carbon dioxide, carbon monoxide and, where bromine is used as a promoter, methyl bromide. In addition, because the reaction is exothermic, the acetic acid solvent is frequently allowed to vaporize to control the reaction temperature and is removed in the gaseous stream. This vapour is typically condensed and most of the condensate is refluxed to the reactor, with some condensate being withdrawn to control reactor water concentration. The portion of the gaseous stream which is not condensed is either vented or passed through a catalytic combustion unit (CCU) to form an environmentally acceptable effluent, as disclosed in publication WO 96/39595. Catalytic combustors have been deployed on TA plants typically upstream of an energy recovery step. Their function is to catalytically combust volatile organic compounds (VOC's) and carbon monoxide and effect complete conversion of any methyl bromide content to HBr and/or Br2. The resulting gas stream can be passed to an energy conversion device, such as an expander, under controlled conditions of pressure and temperature whereby condensation of HBr and/or Br2 is substantially prevented thereby allowing the energy conversion device to be fabricated from relatively inexpensive materials.
In TA production plants power recovery, for example as disclosed in publication 96/39595, is conventionally carried out using an expander at temperatures from about 150-750 ° C., typically 450° C. However, there is scope to improve power recovery using an expander by changes to the configuration of the manufacturing process and the means for recovering power from the process. An improved power recovery system with methods for recovering more power from the gaseous streams of oxidation reactions have been disclosed in publication WO 09/136146. This publication describes an Internal Combustion Open Cycle Gas Turbine (ICOCGT), as disclosed in API 616 Gas Turbines for the Petroleum, Chemical and Gas Industry Services, utilising a standard gas turbine.
The materials of construction for such machines have been developed to avoid corrosion at high temperatures in an oxidative environment and without chemical contamination. Hot section components for land-based turbines are constructed typically in superalloys, protected by coatings that are resistant to oxidation and corrosion which can be overlayed by thermal barrier coatings. The corrosion resistance of the protective coatings arise from their capacity to form protective oxide surface layers at elevated temperatures. Gas turbines generally operate in relatively oxidising gases that contain significant levels of oxygen, typically about 14% WI, However, lower levels of oxygen, such as those in the off-gas from para-xylene oxidation, can prevent or inhibit the formation of protective oxides on coatings. Also, small levels of HBr, up to 100 ppm w/w, can promote the formation of volatile bromides of alloy and coating constituent elements. The off-gas from para-xylene oxidation typically comprises an oxygen concentration less than about 5% w/w, and oxidation catalyst co-factor and by-products comprising organobromides, bromine and acidic bromides.